Highlights
· Blau syndrome is a rare granulomatous autoinflammatory disease typically characterized by a triad of polyarthritis, uveitis, and dermatitis.
· This case underscores the importance of considering Blau syndrome in the differential diagnosis of early-onset hypercalcemia of unknown etiology. Clinical suspicion and early molecular genetic diagnosis are crucial for appropriate treatment, complication surveillance, and genetic counseling for patients and their families.
Introduction
Blau syndrome (OMIM #186580) is a rare autosomal dominant autoinflammatory disorder characterized by a clinical triad of polyarthritis, uveitis, and dermatitis resulting from a heterozygous causative variant in NOD2 [1]. Blau syndrome typically manifests before the age of 5, although an accurate diagnosis may occur later in life. The incidence of Blau syndrome remains undetermined; however, the reported annual incidence of combined granulomatous disorders, including Blau syndrome, early-onset sarcoidosis, and sarcoidosis, among individuals under 18 years of age ranges from 0.06 to 1.02 per 100,000 [2]. Orphanet estimates the prevalence at fewer than 1 in 1,000,000 individuals in the general population.
NOD2 encodes nucleotide-binding oligomerization domain protein 2, which triggers nuclear factor-kappa B (NF-κB) activation and an inflammatory response. The molecular mechanism underlying Blau syndrome remains debated, particularly whether gain-of-function or loss-of-function variants in NOD2 are responsible for the disease phenotype. Granulomatous autoinflammation is the primary pathology in Blau syndrome. Macrophages in granulomatous lesions exhibit 1α-hydroxylase activity, which converts 25-hydroxy vitamin D to 1,25-dihydroxy vitamin D and may cause hypercalcemia [3]. Next-generation sequencing techniques, such as exome and genome sequencing, offer broad genetic diagnostic capabilities. In this report, we present a patient with Blau syndrome and an atypical early presentation of severe hypercalcemia at 11 months of age, before the onset of the classic triad.
Case report
An 11-month-old boy presented with generalized hypotonia, failure to thrive beginning at 10 months of age, constipation at 11 months, and recurrent infections since 7 months of age. The family history was unremarkable (Fig. 1). He had an uneventful perinatal course and was born at 38 weeks’ gestation with a birth weight of 3.65 kg. He had no known medical issues until 7 months of age. His diet consisted of 3 meals of complementary foods (rice, pork, egg, and vegetables), with approximately 10 tablespoons per meal, and 5 formula feedings. His vital signs were as follows: body temperature, 36.8°C; heart rate, 156 bpm; respiratory rate, 44 breaths per minute; and blood pressure, 110/90 mmHg. Physical examination revealed generalized hypotonia, a weight of 6.2 kg (less than 3rd percentile, -3.8 standard deviation [SD] score), a length of 67 cm (less than 3rd percentile, -3.2 SD score), and a head circumference of 47 cm (75th– 90th percentile, 1 SD score). Other findings were unremarkable. Laboratory investigations revealed an elevated serum calcium level (16.3; normal, 8.5–11.0 mg/dL) and a normal serum phosphate level (5.5; normal, 4.8– 8.4 mg/dL).
Further investigations revealed an elevated serum 1,25-dihydroxy vitamin D level (172; normal, 19.9–79.3 pg/mL), a low serum 25-hydroxy vitamin D level (14.8; normal, 20–60 ng/mL), and a suppressed serum intact parathyroid hormone (iPTH) level (3.02; normal, 10–65 pg/mL). A skeletal survey revealed generalized osteopenia with dense sclerotic lines at the bilateral proximal tibiae, as shown in Supplementary Fig. 1. These findings suggested calcitriol-mediated hypercalcemia. Acid-fast staining of gastric lavage samples showed no evidence of tuberculosis. Abdominal ultrasonography revealed bilateral medullary nephrocalcinosis, with no spaceoccupying lesions in the intra-abdominal solid organs. Trio genome sequencing was performed. Hypertension was effectively managed with amlodipine. Severe hypercalcemia was treated with aggressive intravenous fluids and furosemide, but the calcium level remained elevated. Therefore, calcitonin and a short course of systemic corticosteroids were subsequently administered. The timeline of medication doses, duration, and responses is summarized in Fig. 2 and Supplementary Table 1. The serum calcium level normalized, and the patient was discharged with progressively improving muscle tone.
Following discharge, at the age of 1 year and 8 months, the patient gradually developed intermittent fever. Generalized exfoliative dermatitis and abdominal distension also developed after the hypercalcemic episode (Fig. 3). His growth parameters were as follows: weight, 8.6 kg (less than 3rd percentile, -2.5 SD score); length, 70.6 cm (less than 3rd percentile, -4.8 SD score); and head circumference, 48 cm (50th–75th percentile, 0.2 SD score). Abdominal examination revealed hepatomegaly. Musculoskeletal examination demonstrated a limited range of motion in the right wrist, right hip, and both knees. Joint effusions in both knees were confirmed by patellar tap. Plain radiographs of the affected joints showed findings consistent with the previous skeletal survey. Laboratory testing identified anemia and thrombocytopenia. Repeat abdominal ultrasonography performed to evaluate newly identified hepatosplenomegaly revealed marked enlargement of both the liver and spleen without space-occupying lesions, as well as multiple enlarged intra-abdominal lymph nodes. The clinical presentation suggested a systemic infiltrative disease involving the skin and bone marrow.
Trio genome sequencing was performed on genomic DNA extracted when the patient was 11 months old. Whole genome sequencing was performed on the DNBSEQ-T7 platform (BGI, China), achieving an average coverage depth of 30x. Variants were annotated with Variant Effect Predictor version 110, dbNSFP version 4.4a, and our in-house Thai genome databases. Candidate variants were classified according to the recommendations of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology (ACMG/AMP) [4]. We identified a previously reported de novo heterozygous pathogenic missense variant, c.920G>A (p.Arg307Gln), in NOD2 (NM_001370466.1). The variant was classified as pathogenic according to the ACMG/AMP criteria (PS2, PS3_Moderate, PM1, PM5, PP3). This is one of the most common variants reported in patients with Blau syndrome [3,5,6].
A subsequent skin biopsy demonstrated granulomatous dermatitis with focal features consistent with sarcoidal granulomas (Fig. 4). Ophthalmic evaluation revealed anterior uveitis with posterior synechiae in both eyes, a common finding in Blau syndrome. Bone marrow biopsy was performed to investigate the cause of the patient's anemia and thrombocytopenia and revealed nonnecrotizing granulomas in the bone marrow.
After the definitive diagnosis, intravenous methylprednisolone was initiated, followed by a gradual transition to oral prednisolone (2 mg/kg/day). Methotrexate (10 mg/m2 body surface area/wk) was introduced orally as a steroid-sparing immunosuppressive medication. Folic acid supplementation was prescribed concurrently to mitigate potential adverse effects associated with methotrexate therapy. Following initiation of immunosuppressive therapy, the patient's skin involvement improved significantly, and gross motor development progressed. At the last follow-up, his growth parameters were as follows: weight, 13.4 kg (10th percentile, -0.6 SD score); length, 89 cm (less than 3rd percentile, -1.9 SD score); and head circumference, 49 cm (25th–50th percentile, -0.3 SD score). Infliximab therapy, a tumor necrosis factor (TNF)-α inhibitor, was subsequently initiated to improve disease control with the goal of discontinuing corticosteroid treatment.
Written informed consent for publication was obtained from the patient's guardian. Ethics approval was waived by the institutional review board of the Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand.
Discussion
We report a patient with Blau syndrome whose atypical initial presentation was calcitriol-mediated hypercalcemia in the absence of any identifiable acquired cause. The progressive development of additional clinical features subsequently raised suspicion for Blau syndrome. Trio genome sequencing revealed a previously reported pathogenic variant in NOD2 (NM_001370466.1): c.920G> A [p.Arg307Gln].
Reports of Blau syndrome with hypercalcemia are summarized in Table 1 [3,7,8]. In contrast to previous reports, our patient initially presented with failure to thrive and hypercalcemia. None of the classic features of Blau syndrome (arthritis, dermatitis, and uveitis) were detected at disease onset. Unlike the patient reported by Whyte et al. [3] in 2018, our patient had no evidence of musculoskeletal involvement, such as joint pain or an antalgic gait, at the time of hypercalcemia and impaired renal function. Contrary to the earlier report by Whyte et al. [3], plain radiography in our patient revealed no evidence of diffuse osteosclerosis or hyperostosis, findings suggestive of osteopetrosis. This may be explained by increased binding of 1,25-dihydroxy vitamin D to the vitamin D receptor, which can enhance osteoclastmediated bone resorption through NF-κB signaling [8].
Several mechanisms of hypercalcemia in Blau syndrome have been reported. First, NOD2, located in the cytoplasm of blood leukocytes, functions as a microbial sensor that triggers and regulates inflammation. It does so by inducing the translocation of NF-κB to the nucleus, where it facilitates gene transcription [8]. NF-κB is a key transcription factor for osteoclastogenesis, osteoclast activation, and osteoclast function, leading to bone remodeling and the release of calcium from bone into the circulation [8]. Second, granulomas in Blau syndrome contain macrophages that express upregulated 1α-hydroxylase [9]. This enzyme converts 25-hydroxy vitamin D to 1,25-dihydroxy vitamin D, thereby increasing serum calcium through intestinal absorption and bone resorption.
Currently, no standard management recommendations are available for Blau syndrome, and treatment is based primarily on physician experience and case series. Glucocorticoids are the first-line anti-inflammatory treatment, but their effectiveness and adverse effects should be carefully evaluated. Other immunosuppressants, such as methotrexate, hydroxychloroquine, azathioprine, mycophenolate mofetil, and thalidomide, have also been used [10-13]. If patients do not respond, biologic agents such as infliximab or adalimumab (TNF-α inhibitors), anakinra or canakinumab (interleukin-1 blockade), tocilizumab (interleukin-6 blockade), and tofacitinib/baricitinib (Janus kinase inhibitors) should be considered [6,14-16]. Among these agents, TNF-α inhibitors have been reported to significantly alleviate joint and ocular inflammation [5,16,17].
In our patient, serum calcium levels normalized following treatment for Blau syndrome with tapering doses of prednisolone and weekly methotrexate. In addition to hypercalcemia, our patient developed the full spectrum of clinical manifestations associated with Blau syndrome, including the classic triad (dermatitis, arthritis, and uveitis), hepatomegaly, lymphadenopathy, and bone marrow involvement, all of which have been reported in patients with Blau syndrome [5].
Early-onset hypercalcemia without other signs of Blau syndrome can complicate the selection of appropriate genetic testing panels. In this case, we used trio genome sequencing as the initial test, providing comprehensive genetic evaluation for a wide range of singlegene disorders. We advocate comprehensive genetic testing for patients with complex or unusual clinical phenotypes.
In conclusion, Blau syndrome can initially present with calcitriol-mediated hypercalcemia, even in the absence of classic clinical features. Early clinical recognition and prompt molecular genetic diagnosis are crucial for guiding appropriate treatment, monitoring for potential complications, and providing genetic counseling for patients and their families.
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